1. Field of the Invention
The present invention relates to an image sensing apparatus such as an electronic still camera using a solid state image sensing element and in particular to an image sensing apparatus realizing power consumption reduction.
2. Description of the Related Art
The electronic still camera is now enlarging its market among the image sensing apparatuses. Recently, a serious competition is going on to increase the number of CCD pixels and the currently main model uses a CCD unit having 2 millions of pixels or above. In the year of 2000, models using a CCD unit exceeding 3 millions of pixels also appeared on market. Two factors can be considered as the main reason for increasing the number of pixels in the electronic still camera. Firstly, the CCD price has been reduced, the CPU efficiency has been greatly improved such as a Pentium processor (Trademark of Intel corporation), and a large-capacity storage medium has been developed. Accordingly, even a personal computer (hereinafter, referred to as PC) for consumer use can easily handle a large capacity of data.
Secondly, users are already not content with the VGA (640×480 pixels) or an image of one million of pixels which were the main stream a few years ago. Users are requesting for a further fine image. Accordingly, it is expected that the number of pixels will be further increased with more functions and the electronic still camera will find a variety of applications.
However, such an increased number of pixels and functions of the electronic still camera causes a problem of power consumption as the entire system. As the number of pixels is increased, the electrode capacity of the solid state image sensing element is also increased. Moreover, in order to obtain an image having a frame rate identical to the solid state image sensing element having a smaller number pixels, the drive frequency is also increased in proportion to the number of pixels. As a result, the drive circuit of the solid state image sensing element consumes more power and the operation frequency of the entire electronic still camera is also increased, thereby increasing the power consumption. Moreover, as the number of functions of the electronic still camera is increased, the number of peripheral apparatuses such as an LCD apparatus and a recording medium belonging to the entire system is also increased, thereby increasing the power consumption. As a result, the sensing-available time and the number of images that can be recorded are decreased. Thus, the high-performance system cannot exhibit its function sufficiently. That is, for development of the electronic still camera, the reduction of power consumption as a system is also a great problem.